Two major distinctive types of powder feeder currently dominate a market of equipment for thermo or thermal spray operations. One is a rotary disk feeder and the other a so-called “lost weight feeder.”
Rotary disk feeders are considered as almost volumetric type of powder feeding devices. Their performance, however, is highly dependent on the quality of powder filling a metering groove. Depending on particle shape and size of the powders, some feedstock materials exhibit high propensity to bridging and ratholing during groove fill-up. A relevant description of the phenomenon of ratholing is described in Article by T.V. Nguyen entitled Funnel Flow in Hoppers, Journal of Applied Mechanics, December 1980, Vol. 47, pages 729-745, the disclosure of which is herein incorporated by reference. In contrast, other feedstock materials flow more freely and fast. A multitude of groove geometries and corresponding spreader and pick-up shoes has been developed to provide proper performance for a variety of difficult-to-feed materials.
In addition, hoppers with mechanically driven built-in stirrers have been employed to insure that difficult-to-feed powder fills metering groove without large voids or over-compression. However, these types of hoppers are often geometrically complicated and expensive to construct. Still further, they typically require time consuming maintenance, especially during switching from one feedstock powder to another.
Known hoppers also have difficulty regarding how to prevent oversized specks of material (agglomerated or contaminated powder feedstock during improper handling) from entering the feedstock line.
In contrast, a “lost weight feeder” is a fluidized type of feeder that may employ an externally mounted vibrator which is used to maintain the powder in a loose free-flowing state and permeable to the passage of a fluidizing gas. The vibrator shakes the whole hopper that is flexibly mounted on a weight sensor. An output signal from the weight sensor is constantly analyzed and the rate of change is then computed by control circuitry. This device can also utilize a visual display. However, this arrangement has deficiencies; time delay is significant and is a hostage to a desired accuracy. A decrease in the vibrational component of a total force acting on the weight sensor allows to reduce time delay and improves accuracy of computed rate of weight change.
What is desired is an improvement in feeder uniformity by providing a consistently uniform filling of the metering device for a variety of difficult-to-feed materials. It would also be beneficial to improve accuracy and precision of controls, as well as to simplify hopper construction—which would be less costly to manufacture and maintain. An additional improvement would provide more effective protection against oversized specks or foreign objects entering the feedstock line. Yet another benefit would be to reduce the time required to perform hopper disassembly, clean-up and reassembly typically required between changes of feedstock material.